Electromagnetic radiation detecting device comprising an active bolometer and a shielded reference bolometer

ABSTRACT

A device for detecting electromagnetic radiation that comprises an active bolometer provided with a first element sensitive to said electromagnetic radiation and a reference bolometer identical to the active bolometer, provided with a second element sensitive to said electromagnetic radiation. The active bolometer and reference bolometer are arranged close to one another on the same substrate. A cover covers at least the part of the second sensitive element exposed to the electromagnetic radiation and arranges an empty space between said second sensitive element and the cover. The inner wall of the cover is constituted by an absorbent layer made from a material absorbing at least the thermal radiations emitted by the second sensitive element. A reflecting shield forms at least a part of the outer wall exposed to said electromagnetic radiation.

BACKGROUND OF THE INVENTION

The invention relates to a device for detecting electromagneticradiation that comprises:

-   -   an active bolometer equipped with a first element sensitive to        said electromagnetic radiation,    -   a reference bolometer identical to the active bolometer,        equipped with a second element sensitive to said electromagnetic        radiation, said active and reference bolometers being arranged        near to one another on the same substrate,    -   a cover having an outer wall and an inner wall facing towards        the second sensitive element, said cover covering at least the        part of the second sensitive element exposed to the        electromagnetic radiation and arranging an empty space between        said second sensitive element and the cover and,    -   a reflecting shield forming at least a part of the outer wall        exposed to said electromagnetic radiation.

STATE OF THE ART

Bolometers enable the power of electromagnetic radiation in theterahertz range or of infrared rays to be measured by conversion of theradiation into an electric signal. As illustrated in FIGS. 1 and 2, abolometric detector of resistive type comprises a bolometric plate 1suspended above a substrate 2. Bolometric plate 1 is constituted by amembrane 3 absorbing electromagnetic radiation and by a thermistor 4constituted by a heat-sensitive semi-conductor material, the essentialcharacteristic of which is to present a variation of its electricresistance when its temperature varies. Membrane 3 is suspended by meansof thermal insulation arms 5 fixed to substrate 2 by anchorage points 6.Membrane 3 absorbs the incident radiation, transforms the latter intoheat and transmits the heat to a thermistor 4. The temperature rise ofthermistor 4 then induces a voltage or current variation at theterminals of the bolometric detector, measured by a suitable electricassembly. Electrodes (not shown) are for example arranged at the levelof anchorage points 6. In conventional manner, a read circuit isintegrated in substrate 2 and makes use of the measurements made.Thermal insulation arms 5 insulate membrane 3 thermally from substrate 2thereby improving the sensitivity of measurement.

Quality operation of the bolometric detector requires three mainconditions to be met: a low calorific mass, good thermal insulation ofmembrane 3 from substrate 2 and a good sensitivity of the conversioneffect of the temperature rise into an electric signal. The first twoconditions are obtained by implementation of the bolometric detector inthin layers.

Monolithic infrared imagery devices operating at ambient temperature arefabricated by connecting a matrix of bolometric detectors directly to amultiplexing circuit made from silicon of CMOS or CCD type.

The read principle of a bolometric detector consists in measuring thevalue of the resistance of thermistor 4 and the variations thereof. Whenthese variations are very small, in particular for infrared detection(IR), the detected signal is amplified within the read circuit via ahigh-gain amplifier or integrator stage. In general manner, theintegrator stage is privileged as it intrinsically reduces the passbandand thereby eliminates white noise.

In the case of the integrator, the current originating from thebolometric detector rapidly saturates the output signal of a read-outintegrated circuit (ROIC). The useful current containing the informationof the scene then only represents a fraction of the total integratedcurrent. To increase the read sensitivity, the invariant fraction of thecurrent is conventionally shunted to a branch of the circuit called“skimming” branch, which sends only the variable part of the currentcorresponding to the imaged scene to the integrator.

FIG. 3 illustrates the read principle of a bolometer-based detectiondevice. The device comprises an active bolometer 7 which absorbs anincident electromagnetic radiation 8 and enables the latter to bemeasured. The variation of the resistance of active bolometer 7 isrepresentative of the value of this radiation 8. Current read isfrequently used to make this measurement. The current, on output ofactive bolometer 7, comprises a variable fraction and an invariantfraction. The detection device in fact operates in relative manner, i.e.it detects an invariant continuous background signal, which can impingeon measurement of the useful variable signal, which is in general smallcompared with this background signal. It is therefore desirable toeliminate this invariant fraction of the current to obtain optimalmeasurement of the radiation value.

To increase the read sensitivity, the invariant fraction of the currentis preferably shunted in a shunt branch so that only the variable partof the current, called offset current, is sent to an integrator 9. Inelectronics terms, the element that acts as shunt branch has to presenta low noise so as to generate as little disturbance as possible. Forthis, the shunt branch is formed by a resistor of sufficiently highvalue biased by direct injection. What is meant by direct injection isthat the electrons originating from the resistor are injected directlyinto a transistor which performs biasing of the resistor and adjustmentof the detector bias.

A conventional solution consists in using a reference bolometer 10 asshunt branch, i.e. a bolometer 10 that does not detect electromagneticradiation 8.

The shunt branch therefore comprises a reference bolometer 10, asrepresented in FIG. 3, which is blinded by a protective shield 11 placedbetween electromagnetic radiation 8 and reference bolometer 10.Reference bolometer 10 is thus transformed into a passive bolometerwhich does not absorb any radiation and acts as reference.

The efficiency of this type of detection device depends on thecharacteristics of reference bolometer 10 and on its ability to betotally blind.

For example purposes, the document US-A-2007138395 proposes a thermaldetector that comprises a first cell that provides a signal dependent onthe intensity of the incident radiation and a second cell used tocalibrate the output signal of the first cell. Each cell contains aninfrared detector formed on a single substrate and has a cover totallyencapsulating the detector. The second cell is configured to preventtransmission of radiation through the cover and to the detector. Inparticular, the cover has a reflecting screen. The reflecting screen isarranged on the top surface of the cover, the top surface being parallelto the substrate. This reflecting screen enables the second cell to berendered optically opaque. The output signal of the second cell isindependent from the incident radiation and is used as reference signalto calibrate the output signal of the first cell.

Nevertheless, the use of a reflecting screen above the referencestructure is not sufficient to optimally eliminate the offset currentand to detect very weak signals, for example for IR imagery deviceswhich require the use of high IR imagery scene signal detection gains.To obtain a high gain on the detector response, the bolometricresistance is in fact generally biased in pulsed manner.

However, during the biasing pulse, a self-heating phenomenon of thebolometer is observed caused by Joule effect dissipation within thematerial constituting thermistor 4. The offset current then variessignificantly due to this thermal radiation phenomenon.

A subtraction architecture of the offset via a DC current then proves tobe a handicap when resolving the low electromagnetic radiation fluxsignal is involved. The s elf-heating phenomenon induces a saturation ofthe ROIC which limits the usable gain and does not enable optimalperformance of the bolometer to be accessed.

OBJECT OF THE INVENTION

The object of the invention is to provide a detection device enablingreliable and precise measurement of an electromagnetic radiation, inparticular an optimized device enabling an IR imagery scene signal to beamplified as best as possible, with high gains and an improved offsetcurrent skimming.

According to the invention, this object is achieved by anelectromagnetic radiation detection device according to the indexedclaims.

In particular, this object is achieved by the fact that the inner wallof the cover is constituted by an absorbent layer, said layer being madefrom a material absorbing at least the thermal radiations emitted by thesecond sensitive element.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features will become more clearly apparent from thefollowing description of particular embodiments of the invention givenfor non-restrictive example purposes only and represented in theappended drawings, in which:

FIGS. 1 and 2 schematically represent a bolometric detector of the priorart, in top view and perspective view.

FIG. 3 schematically represents the read principle of a bolometer-basedelectromagnetic radiation detection device according to the prior art.

FIG. 4 schematically represents an electromagnetic radiation detectiondevice according to the invention, in cross-section.

FIG. 5 is a schematic representation of the heat exchange principlebetween two surfaces S₁ and S₂.

FIG. 6 schematically represents a particular embodiment of the deviceaccording to the invention, in cross-section.

FIG. 7 schematically represents a detection device according to FIG. 6,in perspective view.

FIGS. 8 to 13 schematically represent the different steps of afabrication method of the detection device according to FIG. 4, incross-section.

FIG. 14 schematically represents a detection device according to aparticular embodiment of the invention, in cross-section.

FIG. 15 schematically represents a detection device according to anotherparticular embodiment, in cross-section.

FIGS. 16 to 21 schematically represent the different steps of afabrication method of the detection device according to FIG. 15, incross-section.

FIG. 22 represents the variation of the current versus time of adetection device according to the invention and of a device of the priorart.

DESCRIPTION OF PARTICULAR EMBODIMENTS OF THE INVENTION

As represented in FIG. 4, a device for detection of an electromagneticradiation 8 comprises an active bolometer 7 provided with a firstelement 12 sensitive to said electromagnetic radiation 8 and a referencebolometer 10 identical to active bolometer 7 and provided with a secondelement 13 sensitive to said electromagnetic radiation 8. What is meantby identical is strictly identical, i.e. that reference bolometer 10 isconstituted by identical materials to those of active bolometer 7 andhas the same architecture and identical dimensions. Reference bolometer10 thus varies with temperature in identical manner.

The active bolometer and reference bolometer, respectively 7 and 10, arearranged on the same substrate 14 and in proximity to one another.

Substrate 14 is a support conventionally made from silicon. It ensuresthe mechanical strength of the active bolometer and of the referencebolometer, respectively 7 and 10, and preferably comprises biasing andread devices of the thermistor resistance (not shown). In particular, inthe case of an IR detection device comprising several microbolometerswith a matrix structure, it can also comprise multiplexing componentsenabling the signals originating from the different microbolometers tobe serialized and to transmit them to a reduced number of outputs inorder to be used by a usual imagery system.

As represented in FIG. 4, active bolometer 7 and reference bolometer 10advantageously have an insulating structure with microbridges, eachstructure comprising anchorage points 6 and thermal insulation arms 5(FIG. 2) in order to minimize heat conduction. The microbridge structurehas a high thermal resistance. A high thermal resistance enables thevariation of the electric resistance to be maximized. The thermal massis adjusted so as to be compatible with the required thermal timeconstant at the level of the system. This structure thereby reduces theresponse time of the signals emitted by the detection device andimproves the signal-to-noise ratio.

The detection device also comprises a cover 15 which covers at least thepart of second sensitive element 13 exposed to electromagnetic radiation8 and arranges an empty space between second sensitive element 13 andcover 15. Cover 15 can be located only above-second sensitive element 13(at the top in FIG. 4), without completely covering reference bolometer10. Cover 15 then creates an open cavity 19. Cover 15 is arranged on theoptic path of electro-magnetic radiation 8, between the radiation sourceand second sensitive element 13. A shielded reference bolometer 10 willthen be referred to, i.e. provided with a cover 15 forming a shield.

Cover 15 has an outer wall 16 and an inner wall 17. Inner wall 17 isfacing towards second sensitive element 13 and outer wall 16 is facingtowards the scene emitting electromagnetic radiation 8.

To achieve optimum operation, active bolometer 7 and reference bolometer10 advantageously have to be identical, or at least comparable, in termsof electric resistance and thermal time constant. What is meant bythermal time constant is the time the bolometer takes to go from onetemperature to another due to the effect of a variation of incidentelectromagnetic radiation 8.

The thermal time constant is the product of the thermal mass by thethermal resistance of the bolometer. What is meant by thermal mass isthe capacity of a material to store thermal energy and to then emit thelatter by radiation (radiative losses), convection or conduction.

The thermal mass and electric resistance are intrinsic characteristicsof the bolometer that depend on the nature of the materials constitutingthe bolometer and on its architecture. The active and referencebolometers, respectively 7 and 10, being identical, the thermal mass andelectric resistance of active bolometer 7 are consequently identical tothose of reference bolometer 10.

The thermal resistance on the other hand depends on the radiative losseswhich quantify terminal heat exchanges between the bolometer and itsenvironment.

More particularly, the thermal resistance comprises two components, afirst component resulting from the heat exchanges between bolometricplate 1 and anchorage point 6, by conduction through thermal insulationarms 5, and a second component depending on the radiative losses whichquantify the heat exchanges of the bolometer with its environment.

The environment of the bolometer determining an amplitude of theradiative heat exchanges consequently influences the value of thethermal time constant.

Each surface in fact, by principle, emits thermal radiations in all thedirections of the space and receives radiations which originate from theother surfaces that surround it. As illustrated in FIG. 5, in the scopeof heat fluxes exchanged by two surfaces S₁ and S₂, the heat exchangesare expressed by the following equation (1):

$\begin{matrix}{\frac{\Phi_{12}}{S_{1}} = {\frac{ɛ_{1}ɛ_{2}}{1 - {\left( {1 - {ɛ_{1}\frac{S_{1}}{S_{2}}}} \right)\left( {1 - ɛ_{2}} \right)} + {ɛ_{1}ɛ_{2}\frac{F_{11}}{F_{12}}}}\left( {\sigma\left( {T_{1}^{4} - T_{2}^{4}} \right)} \right.}} & (1)\end{matrix}$in which:

-   ε₁, and ε₂ are the respective emissivity of surfaces S₁ and S₂,-   φ₁₂ is the density of the radiative flux exchanged between surface    S₁ and surface S₂,-   T₁ and T₂ are the respective temperatures of the two surfaces S₁ and    S₂ and,-   F₁₁ and F₁₂ are the form factors under which S₁ sees itself and sees    S₂.

For a flat surface S₁ as represented in FIG. 5, factor F₁₁ is equal to0.

A conventional bolometer covered only by a case can be respectivelyschematized by surfaces S₁ (flat) and S₂ (FIG. 5). The dimensions of thecase being large both in comparison with the distance from the bolometerto the case and in comparison with those of the bolometer, equation (1)of the heat flux output from bolometer S₁ in the direction of case S₂ issimplified to give the following equation (2):

$\begin{matrix}{\frac{\Phi_{12}}{S_{1}} = {ɛ_{1}\left( {\sigma\left( {T_{1}^{4} - T_{2}^{4}} \right)} \right.}} & (2)\end{matrix}$with S₁/S₂<<1 and F₁₂≈1.

The case located at a distance from the bolometer therefore behaves as ablack surface.

On the other hand, in the case of a reference bolometer 10 present inproximity to the case, the ratio S₁/S₂ being comprised between about 1and 2 is no longer negligible. By entering the value F₁₂ close to 1 andthe ratio S₁/S₂ in equation (1), it can be observed that the presence ofthe case considerably modifies the heat flux density.

The presence of a cover 15 in proximity to reference bolometer 10 and inparticular to second sensitive element 13 can modify the thermalresistance of reference bolometer 10. Furthermore, the influence ofcover 15 on the thermal resistance depends on the distance thatseparates this cover 15 from reference bolometer 10. Active bolometer 7thus sees a different environment from that of reference bolometer 10.The presence of cover 15 explains a difference of thermal resistanceobserved between active bolometer 7 and reference bolometer 10, in spiteof the fact that bolometers 7 and 10 are identical.

According to this principle, provided that the architecture of referencebolometer 10 remains identical to that of active bolometer 7, the onlypossibility for the heat flux exchanged by reference bolometer 10 to beidentical, or at least comparable, with that exchanged by activebolometer 7 is to choose a cover 15 constituted by a material behavingas a black body, i.e. having an emissivity equal to 1 (ε₂=1) and heatedto the same temperature as the focal plane and the case.

Outer wall 16 of cover 15 exposed to electromagnetic radiation 8 alsohas to be reflecting and be constituted by very a good heat-conductingmaterial.

Cover 15 on the one hand has to perfectly absorb the thermal radiationson inner wall 17 facing reference bolometer 10 and on the other hand tobe perfectly reflecting on outer wall 16 of cover 15, facing the sceneemitting electromagnetic radiation 8.

To achieve this, reflecting shield 18 forms at least a part of outerwall 16 exposed to electromagnetic radiation 8. Reflecting shield 18 isconventionally constituted by a reflecting thin layer formed by at leastone material having good reflecting properties and being a good heatconductor.

The whole of outer wall 16 of cover 15 is advantageously constituted byreflecting shield 18 (FIG. 4).

In alternative manner, reflecting shield 18 only forms the part of outerwall 16 exposed to electromagnetic radiation 8. More precisely,reflecting shield 18 only forms the part of outer wall 16 parallel tothe substrate.

Reflecting shield 18 prevents transmission of electromagnetic radiation8 through cover 15 to reference bolometer 10. Reference bolometer 10 isinsensitive to electromagnetic radiation 8.

Reflecting shield 18 is advantageously formed by at least one metalmaterial chosen from aluminum, titanium, gold, nickel, and copper andaluminum alloys.

The detection device is preferably a detection device of IR and ofTerahertz (THz).

Cover 15 presses on substrate 14 and forms with substrate 14 an open orclosed cavity 19 in which reference bolometer 10 is housed.

According to a particular embodiment, cover 15 totally encapsulatessecond sensitive element 13.

As represented in FIGS. 6 and 7, cover 15 can totally encapsulatereference bolometer 10 located in cavity 19. Cavity 19 which is thenclosed is delineated by cover 15 at the top part (at the top of FIG. 6)and by substrate 14 at the bottom part (at the bottom of FIG. 6).

Cover 15 preferably totally covers reference bolometer 10 without beingin contact with the latter. Cover 15 thus forms a capsule which createsa closed and possibly tightly, sealed cavity 19 in which referencebolometer 10 is housed (FIGS. 6 and 7).

Cover 15 can also form a rim pressing on substrate 14 which extends overa part of substrate 14. As illustrated in FIG. 7, the rim is present allaround the capsule improving the adhesion and, eventually, the tightnessof cover 15.

Inner wall 17 of cover 15 is constituted by an absorbent layer 20 sothat the closed cavity 19 constitutes an absorbent cavity. Moreprecisely, the whole of inner wall 17 is constituted by absorbent layer20 to absorb the radiations coming from second sensitive element 13. Theabsorbent material constituting absorbent layer 20 covers the whole ofinner wall 17 of cover 15. Absorbent layer 20 is made from a materialabsorbing at least the thermal radiations emitted by second sensitiveelement 13, preferably by the whole of reference bolometer 10. When thedetection device is in operation, second sensitive element 13 in factheats and emits radiations which are then absorbed by absorbent layer20.

Absorbent layer 20 is advantageously formed by at least one porousabsorbent metallic material chosen from black gold, platinum, silver andchromium. Black gold and platinum are commonly used in the field ofpyroelectric or thermopile detection devices of silver and chromium toelaborate the black absorbers.

Thermalization of the bolometer, i.e. the temperature that referencebolometer 10 will take, is a consequence of the losses by thermalconduction via the thermal insulating arms 5 to which the losses byradiation that are governed by inner wall 17 constituted by a black bodyare added.

According to an alternative embodiment, absorbent layer 20 is formed byat least one polymer material containing graphite. Polymer materialscontaining graphite can absorb up to 80% between 2 μm and 20 μm. Theterm polymer also covers polymer resins of epoxy or polyimide type andphotosensitive resins.

To improve the absorption capacity of absorbent layer 20, the latter canalso be made in the form of a multilayer and/or comprise severalabsorbent materials of different natures.

Reference bolometer 10 having cover 15 described above then presents athermal resistance identical or at least comparable to that of activebolometer 7.

The electromagnetic radiation detection device described above can befabricated in elementary form or on a wafer in collective manner in thescope of a vacuum encapsulation process of the detection devices.

According to the particular embodiments represented in FIGS. 4 and 6,the detection device comprises a non-shielded active bolometer 7 and areference bolometer 10 shielded by cover 15.

A particular embodiment of such a detection device of electromagneticradiation 8 will be described in greater detail with respect to FIGS. 8to 13.

The characteristics of active bolometer 7 and of reference bolometer 10can be slightly changed depending on their fabrication conditions, inparticular for miniaturized IR detection devices. These differences ofcharacteristics then give rise to measurement errors, in particular inthe case of precise measurements of the temperature change of athermistor 4 (FIG. 2). To improve the precision of the measurements andin particular to prevent this type of error, active bolometer 7 andreference bolometer 10 are preferably formed simultaneously on the samesubstrate 14.

As represented in FIG. 8, the detection device fabrication method firstcomprises deposition, on the substrate 14 comprising anchorage points 6,of a first sacrificial layer 21, preferably made from polyimide, with athickness approximately equal to the thickness of anchorage points 6 ofactive and reference bolometers 7 and 10. Bolometric plates 22 and 23respectively comprising first sensitive element 12 and second sensitiveelement 13 are then made in conventional manner to form active andreference bolometers respectively 7 and 10.

As represented in FIG. 9, a second sacrificial layer 24, preferably madefrom polyimide, is then deposited on first sacrificial layer 21 and onbolometric plates 22 and 23. The heat treatments introduced by thisdeposition must not impair the performances of active and referencebolometers 7 and 10. The thickness of second sacrificial layer 24 iscomprised between 0.5 μm and 5 μm so as to form the empty space abovesecond sensitive element 13 and to make cover 15 at a distance abovereference bolometer 10 also comprised between 0.5 μm and 5 μm

In FIG. 10, etching of the first and second sacrificial layers,respectively 21 and 24, is then performed facing the reference bolometer10. A recess 25 is etched according to the required shape of cover 15all around reference bolometer 10. Recess 25 thus forms a continuoustrench that surrounds reference bolometer 10 and passes through thewhole thickness of first and second sacrificial layers, respectively 21and 24.

As represented in FIG. 11, different layers constituting cover 15 arethen successively deposited on second sacrificial layer 24 in the formof thin layers by means of any known method. What is meant by thinlayers are layers having a thickness comprise between 0.005 μm and 1 μm.

The layers are constituted for example by a first metal layer 26deposited on second layer 27 of absorbent material. In all cases, firstand second layers, respectively 26 and 27, are chosen such that cover 15presents the necessary optic and thermal properties to performreflection of electromagnetic radiation 8 and absorption of the thermalradiations emitted by reference bolometer 10 resulting from the powerdissipated by Joule effect when the latter is operating.

Deposition of the thin layers is performed by cathode sputtering, bylow-pressure chemical vapor deposition (LPCVD) or by plasma-enhancedchemical vapor deposition (PECVD).

For example purposes, a second layer 27 of black gold can be produced byevaporation of black gold under reduced nitrogen pressure comprisedbetween 2 mbar and 20 mbar.

A second layer 27 of platinum can be deposited by means of conventionalelectrochemical techniques.

Deposition of a second layer 27 of chromium can be achieved for exampleby sputtering in the presence of an argon (Ar)/oxygen (O₂) mixture underthe following conditions:

-   Pressure comprised between 1 mTorr and 20 mTorr,-   Ar/O₂ ratio comprised between 20:1 and 10:1 and,-   Power per surface unit comprised between 0.12 W/cm² and 2 W/cm².

Likewise, deposition of a second layer 27 of silver can be performed byevaporation in the presence of argon or nitrogen, under a pressurecomprised between 75 mTorr and 750 mTorr.

As represented in FIGS. 4 and 13, first and second layers 26 and 27formed in recess 25 thus constitute the side walls of cover 15 restingon substrate 14. The side walls of cover 15 advantageously have athickness comprised between 0.1 μm and 2.5 μm.

As represented in FIG. 12, etching of layers 26 and 27 is then performedfacing the location of active bolometer 7 in order to form cover 15which is only facing the location of reference bolometer 10. Cover 15 isfor example delineated by chemical or plasma etching methods, or by alift-off process.

Finally, first and second sacrificial layers, respectively 21 and 24,are then etched to obtain the detection device having an activebolometer 7 and a reference bolometer 10 on the same substrate 14 aswell as an integrated cover 15 behaving as a black body (FIG. 13).

Etching of sacrificial layers 21 and 24 is preferably performed in asingle step by any known method. To facilitate this last step, openings(not shown) can be provided in the thickness of cover 15. The size ofthe openings has to be sufficiently small not to disturb the functionsof cover 15. Etching of sacrificial layers 21 and 24 releases closedcavity 19.

According to an alternative embodiment that is not represented, thedetection device comprises a single reference bolometer 10 for severalidentical active bolometers 7.

According to an alternative embodiment represented in FIG. 14, whenreference bolometer 10 has an insulating structure with microbridgescomprising anchorage points 6, cover 15 can press not on substrate 14but on anchorage points 6 of reference bolometer 10. As illustrated inFIG. 14, cover 15 presses on at least two of anchorage points 6 ofreference bolometer 10. Cover 15 and bolometric plate 23 of referencebolometer 10 form an open cavity 28. Open cavity 28 is delineated bycover 15 at the top part (at the top in FIG. 14) and by a plane AA atthe bottom part (at the bottom in FIG. 14). Plane AA passes viaanchorage points 6 forming a support. When anchorage points 6 are in thesame plane as bolometric plate 23, plane AA also passes via bolometricplate 23 (FIG. 14). Cover 15 does not cover reference bolometer 10 butonly second sensitive element 13 without being in contact with thelatter.

The method for producing such a device differs from the previousembodiment in that recess 25 is produced by etching facing anchoragepoints 6 of reference bolometer 10. After etching, first and secondlayers 26 and 27 deposited in recesses 25 constitute the side walls ofcover 15 resting on anchorage points 6 of reference bolometer 10.

This alternative embodiment in particular reduces the overall dimensionsand the volume required to produce cover 15. This alternative embodimentis preferred when etching of first and second sacrificial layers 21 and24 is particularly difficult.

According to another particular embodiment represented in FIG. 15, thedetection device differs from the previous embodiment by the fact thatactive bolometer 7 is encapsulated by a capsule 29 and that cover 15 ofreference bolometer 10 has a multilayer absorbent structure ofFabry-Perrot type. Cover 15 in particular comprises a support layer 30constituted by a material transparent to the electromagnetic radiationemitted by reference bolometer 10 and arranged between absorbent layer20 and reflecting shield 18.

Support layer 30, outer wall 16 and inner wall 17 form a resonantcavity. The resonant cavity absorbs any electromagnetic radiation thatmay be emitted by reference bolometer 10. Support layer 30 isconstituted by a material transparent to the electromagnetic radiationemitted by reference bolometer 10.

Capsule 29 encapsulates active bolometer 7 and is constituted by amaterial transparent to electromagnetic radiation 8 advantageouslyidentical to the transparent material of support layer 30.

As represented in FIG. 15, support layer 30 forms an envelope preferablyhaving an identical shape to that of capsule 29.

Capsule 29 totally envelops active bolometer 7 so as to form a closedcavity 19 b in which active bolometer 7 is housed. Closed cavity 19 b isdelineated at the top part by capsule 29 (at the top in FIG. 15) and bysubstrate 14 at the bottom part (at the bottom in FIG. 15).

Capsule 29 is Transparent to Incident Electromagnetic Radiation.

Absorbent layer 20 is formed by least one resistive absorbent materialhaving a suitable squared resistance preferably chosen from titaniumnitride and nichrome. Absorbent layer 20 forms an absorbent cavity atthe level of cover 15. The resistive absorbent material is placed facingreference bolometer 10 and advantageously has a impedance comprisedbetween 376 and 377 Ω.squared, preferably equal to 376.8 Ω.squared.

The transparent material of capsule 29 and/or of support layer 30 ischosen from single-crystal, polycrystalline or amorphous silicon,single-crystal or polycrystalline germanium and SiGe alloys.

The thickness d of support layer 30 depends on the wavelength λ of theelectromagnetic radiation emitted by reference bolometer 10 and on theoptical index n of support 30 layer according to the following equation(3):d=λ/4n   (3)

Support layer 30 supports reflecting shield 18, which is advantageouslymade from metal. For example, to produce a cover 15 presenting a maximumabsorption at 14 μm, the thickness of a support layer 30 made fromamorphous silicon inserted between an absorbent layer 20 constituted bya resistive absorbent material and a reflecting shield 18, for examplemade from aluminum or an aluminum-based alloy, has to be 1 μm. Such astructure then presents an emissivity of 0.8 around 300K, close to thatof a black body (equal to 1).

This structure is particularly suitable for the vacuummicroencapsulation technology, the materials constituting cover 15 beingcommonly used in this field. The device for detecting electromagneticradiation described in the foregoing can be fabricated in the scope of avacuum encapsulation method.

A particular embodiment of a device for detecting electromagneticradiation comprising an active bolometer 7 and a reference bolometer 10,according to FIG. 15, will be described in greater detail with regard toFIGS. 16 to 21.

As represented in FIG. 16, the method for producing such a detectiondevice differs mainly from the previous embodiment in that recess 25 ismade in such a way as to form a continuous trench surrounding referencebolometer 10 and also a trench surrounding active bolometer 7.

Second layer 27 of absorbent material is then deposited and etched, bymeans of any known method, facing the location of active bolometer 7 toform absorbent layer 20 of cover 15 (FIG. 17).

A third layer (not shown) of transparent material is then deposited soas to cover recess 25 and absorbent layer 20 and is then etched by meansof any known method to form support layer 30.

First layer 26 is then deposited and etched so as to constitutereflecting shield 18 (FIG. 18).

Holes 31 are made, either successively or simultaneously, in capsule 29and cover 15 through reflecting shield 18, support layer 30 andabsorbent layer 20 so as to subsequently form a releasing and pumpingchannel 32 for encapsulation of the active and reference bolometers,respectively 7 and 10 (FIG. 19).

The steps represented in FIGS. 20 and 21 consist in producing a ventingchannel 33 (FIG. 20), by means of a conventional method, and in thenencapsulating active and reference bolometers, respectively 7 and 10, ina vacuum by deposition of one or more sealing layers 34 (FIG. 21). Thesealing layers seal off vent 33 and perform an antireflection functionfor active bolometer 7.

First and second sacrificial layers, respectively 21 and 24, are thenetched by means of any known method, thereby releasing cavities 19 and19 b respectively of cover 15 and of capsule 29.

A getter (not shown) can possibly be incorporated in cavities 19 and 19b respectively of cover 15 and of capsule 29. In this case, the getterhas to be activated, conventionally by heat treatment, before sealing ofbolometers 7 and 10.

The device for detecting electromagnetic radiation according to theinvention enables the self-heating component of the signal delivered bythe active bolometer, noted S_(a), to be eliminated and the signal ofthe scene emitting electromagnetic radiation 8 to be amplified to theutmost. Reference bolometer 10 is identical from an electrothermal pointof view to active bolometer 7 and is blind to electromagnetic radiation8, in particular for IR.

This detection device is promising as it enables the thermal behaviourof active bolometer 7 during the biasing pulse to be reproducedidentically, the signal from reference bolometer 10, noted S_(r),containing the same offset and self-heating signal components. Theradiative environment of reference bolometer 10 emulates that of activebolometer 7 in optimized manner, thereby enabling the offset current tobe optimally subtracted. In this way, once the signals from thesebolometers 7 and 10 have been subtracted, the resulting signal notedS_(u) corresponds solely to the contribution of the scene and will thenbe able to be amplified by the rest of the read chain.

As represented in FIG. 22, the variations of the signal from activebolometer 7 and from reference bolometer 10, respectively S_(a) andS_(r), during the biasing phase, show that the signal from referencebolometer 10 follows that of active bolometer 7. In comparison withsignal S₀ from a reference bolometer only having a metal shieldaccording to the prior art, the presence of an offset current residualof approximately 1% is observed. The integrated signal will then becomposed of a non-useful part not containing information on the scene.

For typical bias currents and gains of conventional IR imagery devices,this residual can reach 30% of the output range of the devices. Theavailable excursion of the signal to image the scene is consequentlyreduced. The distinctive characteristics of the device for detectingelectromagnetic radiation of the invention enable this residual to beeliminated or, at least, greatly reduced. The device for detectingelectromagnetic radiation according to the invention thereby optimizesskimming of the offset current and enables the available dynamic rangeof the scene to be increased.

The invention claimed is:
 1. A device for detecting electromagneticradiation that comprises on a single substrate: an active bolometerequipped with a first element sensitive to said electromagneticradiation, a reference bolometer identical to the active bolometer,equipped with a second element sensitive to said electromagneticradiation, a cover having an outer wall and an inner surface facingtowards the second sensitive element, said cover covering at least thepart of the second sensitive element exposed to the electromagneticradiation and arranging an empty space separating the second sensitiveelement and the inner surface and, a reflecting shield forming at leasta part of the outer wall exposed to said electromagnetic radiation,wherein the entire inner surface of the cover is constituted by anabsorbent layer, said layer being made from a material absorbing atleast the thermal radiations emitted by the second sensitive element. 2.The device according to claim 1, wherein the cover completelyencapsulates the second sensitive element.
 3. The device according toclaim 1, wherein the cover presses on the substrate.
 4. The deviceaccording to claim 3, wherein the cover, with the substrate, forms aclosed cavity in which the reference bolometer is housed.
 5. The deviceaccording to claim 1, wherein the reference bolometer has an insulatingmicrobridge structure comprising anchorage points and the cover presseson at least two of said anchorage points.
 6. The device according toclaim 1, wherein the absorbent layer is a multi-layer.
 7. The deviceaccording to claim 1, wherein the absorbent layer is formed by at leastone porous metallic absorbent material chosen from black gold, platinum,silver and chromium.
 8. The device according to claim 1, wherein theabsorbent layer is formed by at least one polymer material containinggraphite.
 9. The device according to claim 1, wherein the absorbentlayer is formed by at least one resistive absorbent material chosen fromtitanium nitride and nichrome.
 10. The device according to claim 1,wherein the cover comprises a support layer arranged between theabsorbent layer and a reflecting shield and the active bolometer isencapsulated by a capsule, said support layer being constituted by amaterial transparent to the electromagnetic radiation emitted by thereference bolometer.
 11. The device according to claim 10, wherein thetransparent material of the support layer is chosen from single-crystal,polycrystalline or amorphous silicon, single-crystal or polycrystallinegermanium and SiGe alloys.
 12. The device according to claim 1, whereinthe cover is 0.5 μm-5 μm above the reference bolometer.
 13. A device fordetecting electromagnetic radiation that comprises on a singlesubstrate: an active bolometer equipped with a first element sensitiveto said electromagnetic radiation, a reference bolometer identical tothe active bolometer, equipped with a second element sensitive to saidelectromagnetic radiation, a cover having an outer wall and an innerwall facing towards the second sensitive element, said cover covering atleast the part of the second sensitive element exposed to theelectromagnetic radiation and arranging an empty space separating thesecond sensitive element and the inner wall and, a reflecting shieldforming at least a part of the outer wall exposed to saidelectromagnetic radiation, wherein the inner wall of the cover isconstituted by an absorbent layer, said layer being made from a materialabsorbing at least the thermal radiations emitted by the secondsensitive element, and wherein the reference bolometer has an insulatingmicrobridge structure comprising anchorage points and the cover presseson at least two of said anchorage points.